This study is designed to determine, in subjects with Parkinson's Disease, the effects of pallidotomy on motor and cognitive function, as assessed by standard clinical tools, and on gait and rhythmic bimanual motor function, as measured yb specialized tests available. We will relate these to preexisting neurological signs and symptoms, to the stie and size of the pallidal lesion, to the subject's age, and to the duration and severity of presurgical symptoms.